Overload safety device for the drive elements of presses

ABSTRACT

An overload safety device for the drive elements of presses which includes an arrangement for varying the permissible pressure force on the ram of the press in dependence of the position of the ram within its operating stroke. A characteristic curve representing the permissible maximum safe pressure force for each position of the ram throughout its entire operating stroke is depicted with a mechanical or electrical control arrangement operably connected to an overload system to vary the actuation ram pressure of the overload system in dependence on the ram position. In one embodiment a cam surface on a fixed part of the press is arranged generally parallel to the direction of travel of the ram. A control linkage, including a cam part riding on the cam surface, is arranged for movement with the ram. The control linkage includes a piston and cylinder arrangement having a compressed air supply pressure which is varied by movement of the cam part over the cam surface. A second piston and cylinder arrangement is provided which directly communicates with an oil tank arranged in the force train of the ram. This second arrangement includes a switch for relieving the oil pressure in the tank upon a certain predetermined movement of the second piston caused by the oil pressure. The variable compressed air supply is engaged with this second piston to effectively vary the oil pressure, and consequently the ram pressure, which will move the second piston to relieve the oil pressure. Other embodiments, including electrical embodiments may also be utilized.

United States Patent 191 Bergmann et al.

[ July 23, 1974 [73] Assignee: L. Shuler Gmbll, Goppingen,

Germany 221 Filed: June 12, 1972 21 Appl. No.: 261,669.

[30] Foreign Application Priority Data June ll, l97l Germany 2128867 [52] U.S. Cl. 72/20, 72/441 [51] Int. Cl B2lb 37/08 [58] Field of Search..... 72/20, 7, 8, DIG. 5, DlG. 6, 72/DIG. 22, 44l

Primary ExaminerCharles W. Lanham Assistant Examiner-Robert M. Rogers Attorney, Agent, or Firm-Craig &' Antonelli ROTATING DRIVE PARTS CLUTCH [5 7 ABSTRACT An overload safety device for the drive elements of presses which includes an arrangement for varying the permissible pressure force on the ram of the press in dependence of the position of the ram within its operating stroke. A characteristic curve representing the permissible maximum safe pressure force for each position of the ram throughout its entire operating stroke is depicted with a mechanical or electrical control arrangement operably connected to an overload system to vary the actuation ram pressure of the overload system in dependence on the ram position. In one embodiment a cam surface on a fixed part of the press is arranged generally parallel to the direction of travel of the ram. A control linkage, including a cam part riding on the cam surface, is arranged for movement with the ram. The control linkage includes a piston and cylinder arrangement having a compressed air supply pressure which is varied by movement of the cam part over the cam surface. A second piston and cylinder arrangement is provided which directly communicates with an oil tank-arranged in the force train of the ram. This second arrangement includes a switch for relieving the oil pressure in the tank upon a certain predetermined movement of the second piston caused by the oil pressure. The variable compressed air supply is engaged with this second piston to effectively vary the oil pressure, and consequently the ram pressure, which will move the second piston to relieve the oil pressure. Other embodiments, including electrical embodiments may also be utilized.

9 Claims, 1 Drawing Figure PAIENTEDMzamn POWER MEANS CLUTCH ROTATING DRIVE PARTS OVERLOAD SAFETY DEVICE FOR THE DRIVE ELEMENTS OF PRESSES SUMMARY AND BACKGROUND OF THE INVENTION The present invention relates to an overload safety device for protecting the drive elements of extrusion presses and the like. The present invention is particularly related to an arrangement for relieving the forces on such a press in response to the press experiencing a force larger than a predetermined permissible compressive force.

In presses of the type contemplatedby the present invention, it is possible to calculate a nominal-or rated press force for the latter portion of the press stroke starting at a certain position in the stroke path and ending when the press ram is at its dead-center position. This portion of the press stroke is referred to as the nominal press force path. If the nominal press force is exceeded, the stationary and axially reciprocating parts of the press are overstressed with consequent possible damage to the same.

Rotating parts of the press may be overstressed by compressive forces on the press ram which are smaller than the nominal press force discussed above and corresponding to overstress of the stationary and axially reciprocating parts of the press. In the type of presses contemplated by the present invention, the rotating parts are particularly vulnerable to these small compressive forces during that portion of the press stroke which precedesthe nominal press force path. Since the larger compressive forces are usually not experienced by the press ram until late in the press stroke, during the nominal press force for the entire press stroke, and therefore the rotating parts may be arranged for withstanding only lesser forces during the initial portion of the press stroke. 4

It has been proposed to protect the rotating parts by torque limitation devices. However, such torque limitation devices are operable only up to the the beginning of the nominal press force path. In this connection, since, in the nominal press force range proper, even the smallest torques can evoke unduly great forces, the press cannot be adequately protected solely by limiting the torque during this portion of the press stroke.

It has further been proposed to provide overload safety devices which respond to the nominal press force and which are disposed directly in the force train of the press. Such safety devices may include either means for yielding or means for shutting off pressing power when the nominal press force is exceeded. Although this type of device may protect the press during the nominal press force path, it is not effective during the earlier part of the press stroke to protect the rotating parts.

The present invention contemplates the provision of an overload safety arrangement for presses which is operable over the entire operating stroke of the press ram. The present invention further contemplates a specific embodiment of such an overload safety arrangement I for use with an extrusion press of the type wherein the press force is transmitted to the press ram by a pressure medium.

The present invention further contemplates an arrangement for positively controlling the magnitude of the compressive force on the ram in dependence on a predetermined characteristic curve of the force moments over the entire operating stroke of the drive piston rod. This characteristic curve may be determined by testing and or calculating the maximum safe compressive force against the press ram for each position of the press ram throughout its operating stroke. 7

The present invention contemplates a specific embodiment utilizing a fixed control cam having a cam surface configured to correspond to the characteristic curve for the press. This specific embodiment further includes a linkage having a portion guided along the control cam and another portion operatively connected to a pressure control device for adjusting the control device in response to the movement over the control cam. The linkage is operatively connected to the press ram so as to move along the control cam as the press ram traverses the press stroke path. The pressure control means contemplated by the present invention includes a fluid pressure transmitter or intensifier. This transmitter is connected on one side with a pressure oil tank arranged in the line of force of the press ram and on the other side with compressed air means. This pressure exerted by the compressed air means on the transmitter is controlled by the linkage and control cam configuration. A pressure medium amplifier is contemplated for amplifying the pressure changes in the compressed air supply to the transmitter. A cylinder-piston arrangement can serve as the pressure medium amplifier, the cylinders or pistons of which are movable by the linkage, and the pressure medium of which is effective on the pressure transmitter. The compressed air means of the pressure transmitter can include a piston having one side under the effect of a compressed air supply and the other side in communication with the pressure medium of the amplifier.

The present invention also contemplates other embodiments where the required course of the characteristic curve is simulated in an electrical fashion. For example, a computer could be programmed with the characteristic curve and could control the relief valve for an oil pressure supply in the press ram force train in such a manner as to relieve the press ram when the compressive forces exceed the value on the characteristic curve.

The present invention also contemplates an electrical or electronic embodiment where the characteristic curve is represented as a first variable current signal to one side of a comparator and where the compressive force experienced by the press is representing as a second variable current signal to the other side of the comparator. The comparator includes means for sending an output signal to relieve the compressive force in response to a predetermined difference in the first and second signals. The output signal of the comparator could shut off the press or effect the opening of a relief valve to relieve an oil pressure tank arranged in the line of force of the press ram. The second signal could be obtained by a pressure detector arranged to detect the compressive force of the ram. This pressure detector could be connected to a fluid pressure tank arranged in the line of force of the ram or it could directly measure the compressive force of the ram by strain gages or the like. The first signal could be obtained by a variable resistor arrangement connected to a plunger riding in a cam surface corresponding to the characteristic curve for the press. The first signal could alternatively be obtained from a variable resistor or like arrangement coupled to a rotating part of the drive mechanism.

The present invention further contemplates embodiments where the characteristic curve is correlated to the crank angle of the main press shaft, rather than to direct liner movement of the ram. The present invention also contemplates stepwise control of the pressure oil tank via an arrangement of several step switches. Such a stepwise control would permit continuation of the press stroke after the reaching of a temporary overload so as to preclude inadvertent stoppage of the press ram for stress peaks of very short duration. The several step switches could be formed by mechanical or electrical means including a plurality of pressure tanks and related valving.

The present invention also contemplates embodiments which effect control of a main clutch or power means of the press rather than control by relief of a hydraulic medium arranged in the line of force of the press ram.

Other embodiments utilizing the basic concepts of the presention are also contemplated. An important feature of all contemplated embodiments is the continuous control of the compressive force throughout the operating stroke of the press ram.

BRIEF DESCRIPTION OF THE DRAWING vention.

Since the invention can be used on many types of presses and further since the construction of such presses is known, the drawing includes only those portions of a press necessary for understanding and practicing the invention. It will be understood that the drive piston rod is operatively driven by other press parts including a rotating press shaft. Also, power means for driving the press ram and clutch means for engaging and disengaging the power means are arranged in a known manner for reciprocably driving the piston rod 5 and associated ram structure. Reference is made to the assignee of the present applications handbook, pages 182-188 and 195-204 for further information related to press arrangements with which the present invention can be utilized.

DETAILED DESCRIPTION OF THE DRAWING A press ram 1 contains a pressure oil container 2. In the upper portion of this container 2, a piston 4 is guided which is attached to a socket joint 3. A drive piston rod 5 of the press drive is articulated to the socket joint 3 by a pin 6. The movement of the socket joint 3 is limited in the downward direction by an end face 7 of the pressure oil tank 2 and in the upward direction by a flange ring 8 attached to the press ram 1. During the return or upward stroke, the press ram 1 is lifted by the socket joint 3 entrained by the drive piston rod 5 via the pin 6 and contacting with its topside the underside of the flange ring 8. During the operating or downward stroke, the socket joint 3 is in contact with the flange ring 8 due to the pressure in the pressure oil container 2, by means of which the socket joint 3 is pressed upwardly by way of the piston 4. When the force exerted on the bottom face of the socket ram 1 exceeds the force of the oil pressure effective on the piston 4, then the press ram comes to a standstill, while the drive piston rod 5, the socket joint 3, and the piston 4 still move further downwardly by a minor amount because of the gap between end face 7 and socket joint A pressure transmitter or hydraulic intensifier 10 forms a pressure control device which is in communication with an oil conduit 9 fed by an oil pump 26 or the like and leading to the pressure oil tank 2. A piston 12 guided in a cylinder 11 of the pressure transmitter 10 is on one side 13 under the effect of compressed air and its piston rod 14 is guided in a bushing 15 connected to the oil conduit 9. A compressed air supplying means 27, such as a pump or supply tank, normally maintains a high pressure compressed air supply to side 13. The compressed air is under such a high pressure that it corresponds, together with the pressure ambient in the oil line 9, to the nominal press force or to the maximum force permissible for the press or tool, or ranges somewhat below these forces.

Upon a downward movement of the press ram 1 and the piston 4 when the nominal press force has been exceeded, the pressure of the pressure transmitter 10 and the oil pressure is overcome and the piston 12 and the piston rod 14 of the pressure transmitter 10 are pressed upwardly. A microcontact 28 opening a magnetic valve 29 is provided which is automatically switched directly after onset of this movement, so that the air in the pressure transmitter escapes. The piston 12 and piston rod 14 are then completely pushed back by the oil in line 9 causing release of the oil backfiow under piston 4 whereby simultaneously the drive piston rod 5 is relieved. Alternatively, as shown in dashed lines, the contact 28 can effect disengagement of a clutch 30 of the drive train.

With only the above described elements, the overload safety device responds always only in those cases where the press ram 1 encounters a resistance exceeding the nominal press force.

In order to be able to control the pressure transmitter according to this invention during the entire operating stroke, so that the overload safety device responds at any time at a preselected press force of the press ram 1, a fixed control cam 17, attached, for example, at the press frame 16, and a cylinder-piston arrangement 18 are provided, by means of which a load corresponding to the characteristic maximum safe force curve for the entire operating stroke can be exerted on the pressure transmitter 10. The control cam 17 is configured in correspondence with the characteristic force curve for the entire operating stroke of the press being used. A roller 22 disposed at a rod 19 of the piston 21 guided in the cylinder 20 is in constant contact with the control cam 17 due to compressed air effective on the piston side 23 facing away from the rod 19. This piston side 23 is connected, via a conduit 24, with the underside 25 of the piston 12 of the pressure transmitter 10, which latter, just as the cylinder-piston arrangement 18, is mounted for movement with the press ram 1. When the roller 22, during the operating stroke of the press ram 1, passes from the approximately linearly extending section of the control cam 17 into the curved lower section, the roller 22, rod 19, and piston 21 are moved toward the right to an increasing extent. The pressure at the underside 25 of the pressure transmitter piston 12 is then reduced due to an enlargement of the cylinder space defined by the piston side 23. Since a relatively constant compressed air supply is supplied to side 13 of the piston 12, the resultant force on the piston 12 in the downward direction is increased thereby raising the pressure in oil container 2 and line 9.

Accordingly, in correspondence with the configuration of the control cam 17, the force effective on the piston rod 14 is varied during the press stroke, which force results from the difference of the forces effective on the piston 12. These forces are the constant force on the piston side 13 and the force on the underside 25 variable via the control cam in dependence on the change in volume in the cylinder-piston arrangement 18.

The configuration of the control cam 17 depends on g the predetermined permissible load of individual drive parts for the various ram positions and/or for blank holder position in cases where the blank holder is moved.

The control cam 17, by means of which the press can be provided with any desired force-path characteristic, is designed in the present embodiment in such a manner that the pressure transmitter first is under a light load, and is then under an ever-increasing load until the beginning N of the nominal press force path. The load on the pressure transmitter attained upon reaching this point N corresponds to the nominal press force. Due to the vertical extension of the control cam from N to the lower dead-center position U.T., this force relationship remains preserved. This cam configuration protects a crank press or an eccentric press against overload even up to the beginning of the nominal press force path, since in this stroke range the permissible press force is smaller than the nominal press force, due to the kinematics of mechanical press drives.

This property of being able to control the response pressure of the pressure control device as desired in accordance with the ram position and force, not exhibited by conventional, uncontrollable pressure control devices, is particularly advantageous, for example, when driving the extrusion ram of sheet metal extrusion presses. In these machines, for example, extrusion forces due to high workpieces, which forces deviate from the customary force characteristic of the ram forces, must not be exceeded. That is, extrusion forces less than the nominal press force but greater than the permissible press force for the earlier portion of the press stroke path may be experienced due to the high workpieces. In order to prevent wear and tear on the extrusion tools and on the extrusion ram, the overload safety device of the present invention can readily respond prior to reaching the nominal press force path.

This invention is also advantageous for other types of press drives wherein the load bearing capacity varies considerably during the stroke, due to the kinematic relationships. For example, the present invention would also be useful in blank holder drives in extrusion presses.

The present invention can also be utilized with the same advantages discussed above in hydraulic presses.

While we have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to those skilled in the art, and we therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and 6 modifications as are encompassed by the scope of the appended claims.

We claim:

1. An overload safety arrangement for use with presses of the type having a first part and a second part, said second part being a relatively movable ram means, wherein the ram means experiences ram pressure force during an operating stroke as it moves from a first position with respect to the first part to a second position with respect to the first part; said arrangement comprising: characteristic curve means for depicting respective predetermined maximum safe ram pressures for all positions of the ram means during an operating stroke, pressure force detecting means for detecting the ram pressure forces actually experienced by the ram means, and control means operatively connected to the characteristic curve means and the pressure force detecting means and including pressure force reducing means for reducing the ram pressure forces whenever the detected ram pressure forces exceed the respective maximum safe ram pressure force depicted by the characteristic curve means to the particular position in the operating stroke that the ram means is experiencing, said characteristic curve means including a cam surface on said first part which extends over a distance corresponding to the operating stroke of the ram means, a linkage which is movable along with the ram means, said linkage having a camming part at one end thereof which is in engagement with said cam surface, the other end of said linkage being operatively connected to the control means, said reducing means including means for relieving fluid pressure in a fluid tank means arranged in the force train of the ram means, said detecting means including means in direct communication with said fluid tank means, said control means including a pressure control device, said pressure control device including a first piston having a first end face directly communicating with the fluid pressure in said fluid tank means, and compressed air means operatively engage the first piston to force the first piston oppositely of the direction of force of the fluid pressure from said tank means.

2. An arrangement according to claim 1, wherein said reducing means includes means for reducing the force of the compressed air means on said first piston.

3. An arrangement according to claim 2, wherein said reducing means includes switch means responsive to movement of said first piston to actuate a valve means for exhausting a portion of the compressed air supplied to the first piston.

4. An arrangement according to claim 1, wherein said compressed air means includes a constant air pressure force on a second face of said first piston and a variable air pressure force on a third face of said first piston, said second face facing oppositely to said first face and said third face facing the same direction as said first face, and wherein said air pressure on said third face is varied in response to movement of the linkage camming partover the cam surface.

5. An arrangement according to claim 4, wherein a cylinder with a second piston reciprocating therein is arranged between the camming part and the pressure control device, one side of said second piston being fixedly connected to a rod which is attached to said camming part, the other side of said second piston being in communication with the compressed air impinging on the third face of the first piston.

8. An arrangement according to claim 1, wherein the predetermined safe ram pressure forces vary for different positions of the ram means during the operating stroke.

9. An arrangement according to claim 8, wherein the predetermined forces are greater for the later stages of the operating stroke than they are for'the earlier stages. 

1. An overload safety arrangement for use with presses of the type having a first part and a second part, said second part being a relatively movable ram means, wherein the ram means experiences ram pressure force during an operating stroke as it moves from a first position with respect to the first part to a second position with respect to the first part; said arrangement comprising: characteristic curve means for depicting respective predetermined maximum safe ram pressures for all positions of the ram means during an operating stroke, pressure force detecting means for detecting the ram pressure forces actually experienced by the ram means, and control means operatively connected to the characteristic curve means and the pressure force detecting means and including pressure force reducing means for reducing the ram pressure forces whenever the detected ram pressure forces exceed the respective maximum safe ram Pressure force depicted by the characteristic curve means fo the particular position in the operating stroke that the ram means is experiencing, said characteristic curve means including a cam surface on said first part which extends over a distance corresponding to the operating stroke of the ram means, a linkage which is movable along with the ram means, said linkage having a camming part at one end thereof which is in engagement with said cam surface, the other end of said linkage being operatively connected to the control means, said reducing means including means for relieving fluid pressure in a fluid tank means arranged in the force train of the ram means, said detecting means including means in direct communication with said fluid tank means, said control means including a pressure control device, said pressure control device including a first piston having a first end face directly communicating with the fluid pressure in said fluid tank means, and compressed air means operatively engage the first piston to force the first piston oppositely of the direction of force of the fluid pressure from said tank means.
 2. An arrangement according to claim 1, wherein said reducing means includes means for reducing the force of the compressed air means on said first piston.
 3. An arrangement according to claim 2, wherein said reducing means includes switch means responsive to movement of said first piston to actuate a valve means for exhausting a portion of the compressed air supplied to the first piston.
 4. An arrangement according to claim 1, wherein said compressed air means includes a constant air pressure force on a second face of said first piston and a variable air pressure force on a third face of said first piston, said second face facing oppositely to said first face and said third face facing the same direction as said first face, and wherein said air pressure on said third face is varied in response to movement of the linkage camming part over the cam surface.
 5. An arrangement according to claim 4, wherein a cylinder with a second piston reciprocating therein is arranged between the camming part and the pressure control device, one side of said second piston being fixedly connected to a rod which is attached to said camming part, the other side of said second piston being in communication with the compressed air impinging on the third face of the first piston.
 6. An arrangement according to claim 1, wherein the pressure force reducing means includes means for reducing the pressure in a hydraulic cushion means arranged in the ram means.
 7. An arrangement according to claim 1, wherein the pressure force reducing means includes means for reducing the pressure in a fluid-medium operated clutch of the drive train of the ram means.
 8. An arrangement according to claim 1, wherein the predetermined safe ram pressure forces vary for different positions of the ram means during the operating stroke.
 9. An arrangement according to claim 8, wherein the predetermined forces are greater for the later stages of the operating stroke than they are for the earlier stages. 